Tuomioja has been a member of the Finnish Parliament 1970–1979 and 1991–present. He held the position of Minister of Trade and Industry in Lipponen's 2nd government, and became the Minister for Foreign Affairs after Tarja Halonen was elected the President of Finland.[4]

Tuomioja, like several other Finnish socialist politicians of today, took part in the illegal occupation of the Old Student House (Vanha ylioppilastalo) in Helsinki on 25 November 1968. He was member of the anti-war group Committee of 100 of Finland and took part in the so-called Erik Schüller case, in which a group of students made public incitement against obligatory conscription. Despite his anti-war stance, Tuomioja did carry out his own mandatory military service and is a reservist staff sergeant.[3]

Tuomioja is the author of several books. His A Delicate Shade of Pink about his grandmother Hella Wuolijoki and her sister Salme Murrik won the Non-Fiction Finlandia Prize in 2006. The book was originally written in English and translated to Finnish as Häivähdys punaista.[2]